Designer HF-based fluorination reagent: highly regioselective synthesis of fluoroalkenes and gem-difluoromethylene compounds from alkynes

gem-Difluorination of carbon-carbon triple bonds using Brønsted acid/Bu4NBF4 or electrogenerated acid

gem-Difluorination of carbon-carbon triple bonds was conducted using Brønsted acids, such as Tf2NH and TfOH, combined with Bu4NBF4 as the fluorine source. The electrochemical oxidation of a Bu4NBF4/CH2Cl2 solution containing alkyne substrates could also give the corresponding gem-difluorinated compounds (in-cell method). The ex-cell electrolysis method was also applicable for gem-difluorination of alkynes.

Determination of the Hammett Acidity of HF/Base Reagents

Harnessing the acidity of HF/base reagents is of paramount importance to improve the efficiency and selectivity of fluorination reactions. Yet, no general method has been reported to evaluate their acidic properties, and experimental designs are still relying on a trial-and-error approach. We report a new method based on 19F NMR spectroscopy which allows highly sensitive measures and short-time analyses. Advantageously, the basic properties of the indicators can be determined upstream by DFT calculations, affording a simple yet robust semiempirical approach. In particular, the indicators used in this study were rationally designed to fit on the conceptually appealing and commonly used Hammett scale. This method has been applied to commercially available and recently developed HF/base reagents.

Transition-Metal-Free Approach for Z-Vinyl Fluorides by Hydrofluorination of Alkynes bearing SF4 and SF5 Groups

The synthesis of vinyl fluorides plays a crucial role in various scientific disciplines, including pharmaceutical and materials sciences. Herein, we present a direct and stereoselective hydrofluorination method for the synthesis of Z isomers of vinyl fluorides from alkynes containing unexplored SF5 and SF4 groups. Our strategy employed tetrabutylammonium fluoride (TBAF) as a fluorine source. It demonstrates high compatibility with aryls, biaryls, heteroaryls, and tert-alkyl groups, allowing facile incorporation of SF5 and SF4 groups across the triple bond without any transition-metal catalysts. This approach avoids the potential decomposition of the SF5 or SF4 units via coordination with transition metals or acidic protic sources. Remarkably, this transformation proceeded at room temperature without any additional additives, providing the Z isomer of vinyl fluorides in excellent yield and high selectivity. The presence of a water molecule as a hydrate in TBAF is essential for efficient conversion. This methodology opens new avenues for the synthesis of enchanting SF5 - and SF4 -containing fluorinated vinylic scaffolds, thereby providing advanced opportunities for novel drug discovery and fluorinated polymers.

Synthesis of 2,2-difluoro-1,3-diketone and 2,2-difluoro-1,3-ketoester derivatives using fluorine gas

Solutions of 1,3-diketones and 1,3-ketoester derivatives react with fluorine to give the corresponding 2,2-difluoro-1,3-dicarbonyl derivatives in the presence of quinuclidine. Quinuclidine reacts with fluorine in situ to generate a fluoride ion that facilitates limiting enolization processes, and an electrophilic N-F fluorinating agent that is reactive towards neutral enol species.

Catalytic HF Shuttling between Fluoroalkanes and Alkynes

In this paper, we report BF3  ⋅ OEt2 as a catalyst to shuttle equivalents of HF from a fluoroalkane to an alkyne. Reactions of terminal and internal aliphatic alkynes led to formation of difluoroalkane products, while diarylalkynes can be selectively converted into fluoroalkenes. The method tolerates numerous sensitive functional groups including halogen, protected amine, ester and thiophene substituents. Mechanistic studies (DFT, probe experiments) suggest the catalyst is involved in both the defluorination and fluorination steps, with BF3 acting as a Lewis acid and OEt2 a weak Lewis base that mediates proton transfer. In certain cases, the interconversion of fluoroalkene and difluoroalkane products was found to be reversible. The new catalytic system was applied to demonstrate proof-of-concept recycling of poly(vinylidene difluoride).

Hydrofluorination of Alkynes: From (E) to (Z)

The hydrofluorination of alkynes is an efficient synthetic route to monofluoroalkenes or difluoroalkanes. Both fluorinated motifs have found applications in medicinal chemistry and beyond. This review explores the recent advances in the hydrofluorination of diverse alkynes through various activation methods, from classical coinage metal catalysis to metal-free conditions. The range of alkynes goes from the simplest unactivated alkynes to activated ones (ynones and derivatives, ynamides, alkynyl sulfides and sulfones as much as haloalkynes). Regio- and stereoselective methods exists, but there is still room for improvement depending on the type of alkyne.

Fluorinated Imines in Tandem and Cycloaddition Reactions

The chemistry of fluorinated compounds has experienced extraordinary growth in recent decades due to the many and varied properties which many of the compounds that contain fluorinated groups possess. Among all of them, fluorinated chiral imines, in particular the Ellman's imines, are of great importance since they are some of the most interesting building blocks for the synthesis of a large number of enantioenriched carbocycles and heterocycles with extraordinary biological and synthetic properties. This personal account covers the most significant results obtained in our research group in the last two decades concerning asymmetric tandem reactions, paying special attention to the intramolecular aza-Michael reaction (IMAMR), diversity oriented synthesis (DOS), asymmetric tandem reactions involving a p-tolylsulfinyl group as chiral inducer and cycloaddition processes, in particular, the Pauson-Khand reaction, [2+2+2]-cycloadditions and metathesis reactions, starting mainly from enyne compounds and through the use of fluorinated chiral N-sulfinyl imines and their derivatives as starting materials.

Au(I)-Catalyzed Regioselective Hydrofluorination of Propargylamines Using Aqueous HF

Gold-catalyzed regioselective hydrofluorination of alkynes using aqueous HF has been achieved by employing an amide directing group. For both aryl- and alkyl-substituted propargylamines, the fluorination occurred at the site distal to the amino group.

Room temperature deoxofluorination of aromatic aldehydes with XtalFluor-E under highly concentrated conditions

The synthesis of difluoromethyl-containing compounds exploiting the deoxofluorination reaction of aromatic aldehydes using XtalFluor-E is described. This transformation occurs at room temperature under highly concentrated conditions, i.e., with no added solvent. A wide range of difluoromethyl-containing compounds was obtained in 21 to 87% isolated yields.

Gold catalyzed hydrofluorination of propargyl alcohols promoted by fluorine-hydrogen bonding

The hydroxyl group was discovered to promote gold catalyzed hydrofluorination of alkynes via hydrogen bonding interaction. Based on this strategy, propargyl alcohols could be hydrofluorinated smoothly using Et₃N·3HF under acidic additive-free conditions, which provided a straightforward alternative protocol for the synthesis of 3-fluoroallyl alcohols.

From cyclic (alkyl)(amino)carbene (CAAC) precursors to fluorinating reagents. Experimental and theoretical study

Addition of anhydrous HF to the hydrochloride [^MeCAACH][Cl(HCl)_0.5] resulted in the formation of salts with high HF content. By stepwise removal of HF in vacuo, we selectively prepared [^MeCAACH][F(HF)₂] (3) and [^MeCAACH][F(HF)₃] (4). We also characterised a salt with [F(HF)₄]^- anions within the structure of [^MeCAACH][F(HF)_3.5] (5). Compounds with a lower content of HF were not accessible under vacuum conditions. ^MeCAAC(H)F (1) was selectively prepared by abstraction of HF from 3 with CsF or KF, while [^MeCAACH][F(HF)] (2) was prepared by mixing 3 and 1 in a 1 : 1 ratio. Compound 2 proved to be quite unstable as it tends to disproportionate into 1 and 3. This observation triggered our computational study, in which the structural relationships between CAAC-based fluoropyrrolidines and dihydropyrrolium fluorides were investigated using different DFT methods. The study showed that the results were very sensitive to the computational method used. For a correct description, the quality of the triple-ζ basis set was crucial. Surprisingly, the isodesmic reaction of [^MeCAACH][F] + [^MeCAACH][F(HF)₂] → [^MeCAACH][F(HF)] + [^MeCAACH][F(HF)] did not confirm the low thermodynamic stability of 2. Furthermore, the use of 3 as a nucleophilic fluorinating reagent was tested on a range of organic substrates, as it is the most stable compound in this series. It was found to have the potential to fluorinate benzyl bromides, 1- and 2-alkyl bromides, silanes and sulfonyls with good to excellent yields of the target fluorides.

Integrating I(I)/I(III) catalysis in reaction cascade design enables the synthesis of gem-difluorinated tetralins from cyclobutanols

Partially saturated, fluorine-containing rings are ubiquitous across the drug discovery spectrum. This capitalises upon the biological significance of the native structure and the physicochemical advantages conferred by fluorination. Motivated by the significance of aryl tetralins in bioactive small molecules, a reaction cascade has been validated to generate novel gem-difluorinated isosteres from 1,3-diaryl cyclobutanols in a single operation. Under the Brønsted acidity of the catalysis conditions, an acid-catalysed unmasking/fluorination sequence generates a homoallylic fluoride in situ. This species serves as the substrate for an I(I)/I(III) cycle and is processed, via a phenonium ion rearrangement, to an (isolable) 1,3,3-trifluoride. A final C(sp³)-F bond activation event, enabled by HFIP, forges the difluorinated tetralin scaffold. The cascade is highly modular, enabling the intermediates to be intercepted: this provides an expansive platform for the generation of structural diversity.

Strategies for Nucleophilic C(sp3)-(Radio)Fluorination

This Perspective surveys the progress and current limitations of nucleophilic fluorination methodologies. Despite the long and rich history of C(sp³)-F bond construction in chemical research, the inherent challenges associated with this transformation have largely constrained nucleophilic fluorination to a privileged reaction platform. In recent years, the Doyle group─along with many others─has pursued the study and development of this transformation with the intent of generating deeper mechanistic understanding, developing user-friendly fluorination reagents, and contributing to the invention of synthetic methods capable of enabling radiofluorination. Studies from our laboratory are discussed along with recent developments from others in this field. Fluoride reagent development and the mechanistic implications of reagent identity are highlighted. We also outline the chemical space inaccessible by current synthetic technologies and a series of future directions in the field that can potentially fill the existing dark spaces.

Metal-Free Electrochemical Oxidative Difluoroethylation/Cyclization of Olefinic Amides To Construct Difluoroethylated Azaheterocycles

A new strategy of electrochemical oxidative difluoroethylation to generate difluoroethyl radical with sodium difluoroethylsulfinate (DFES-Na) has been reported for the first time. The method allows quick access to a variety of valuable difluoroethylated azaheterocycles including oxindoles and isoquinoline-1,3-diones via radical tandem difluoroethylation/cyclization in moderate to good yields. The electrochemical cyclopropyldifluoromethylation of N-arylacrylamides also works well using this strategy. Moreover, radical capture and cyclic voltammetry (CV) experiments are also carried out to determine the proposed mechanism.

Regio- and Enantioselective Intermolecular Aminofluorination of Alkenes via Iodine(I)/Iodine(III) Catalysis

The regio- and enantio-selective, intermolecular vicinal fluoroamination of α-trifluoromethyl styrenes has been achieved by enantioselective II /IIII catalysis. Leveraging C2 -symmetric resorcinol-based aryl iodide catalysts, it has been possible to intercept the transient iodonium intermediate using simple nitriles, which function as both the solvent and nucleophile. In situ Ritter reaction provides direct access to the corresponding amides (up to 89 % yield, e.r. 93 : 7). This main group catalysis paradigm inverts the intrinsic regioselectivity of the uncatalyzed process, thereby providing facile access to tertiary, benzylic stereocenters bearing both CF3 and F groups. Privileged phenethylamine pharmacophores can be generated in which there is complete local partial charge inversion (CF3δ- /Fδ- versus CH3δ+ /Hδ+ ). Crystallographic analyses of representative β-fluoroamide products reveal highly pre-organized conformations that manifest the stereoelectronic gauche effect.

Platinum-Catalyzed Hydrofluorination of Alkynes: Hydrogen Bonding to Indolylphosphine Ligands to Provide Fluoride Reactivity

The reaction of the Pt complexes cis-[Pt(CH3 )2 {R2 P(Ind)}2 ] (Ind=2-(3-methyl)indolyl, R=Ph (1 a), 4-FC6 H4 (1 b), 4-CF3 C6 H4 (1 c)) with HF afforded the fluorido complexes trans-[Pt(F(HF)2 )(CH3 ){R2 P(Ind)}2 ] 2 a-c, which can be converted into trans-[Pt(F)(CH3 ){R2 P(Ind)}2 ] (3 a-c) by treatment with CsF. Addition of 3-hexyne to 2 a-c gave alkyne complexes trans-[Pt(C,C-η2 -C2 H5 C≡CC2 H5 )(CH3 ){R2 P(Ind)}2 {F(HF)2 }] (4 a-c) at which a fluoride is stabilised as polyfluoride in the coordination sphere by hydrogen bonding to the indolyl-substituted phosphine ligands. Subsequent heating of a solution of 4 a in the presence of PVPHF led to fluoroalkene formation. Selective catalytic hydrofluorination of alkynes to yield (Z)-fluoroalkenes were developed. The ability of hydrogen bonding to polyfluoride favours the fluorination step as demonstrated by studies with complexes bearing no indolyl groups at the phosphine ligands.

Au(I) Catalyzed HF Transfer: Tandem Alkyne Hydrofluorination and Perfluoroarene Functionalization

HF transfer reactions between organic substrates are potentially useful transformations. Such reactions require the development of catalytic systems that can promote both defluorination and fluorination steps in a single reaction sequence. Herein, we report a catalytic protocol in which an equivalent of HF is generated from a perfluoroarene | nucleophile pair and transferred directly to an alkyne. The reaction is catalyzed by [Au(IPr)NⁱPr₂] (IPr = N,N′-1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). HF transfer generates two useful products in the form of functionalized fluoroarenes and fluoroalkenes. Mechanistic studies (rate laws, KIEs, density functional theory (DFT) calculations, competition experiments) are consistent with the Au(I) catalyst facilitating a catalytic network involving both concerted S_NAr and hydrofluorination steps. The nature of the nucleophile impacts the turnover-limiting step. The cS_NAr step is turnover-limiting for phenol-based nucleophiles, while protodeuaration likely becomes turnover-limiting for aniline-based nucleophiles. The approach removes the need for direct handling of HF reagents in hydrofluorination and offers possibilities to manipulate the fluorine content of organic molecules through catalysis.

Electrochemical Fluorination of Vinyl Boronates through Donor-Stabilized Vinyl Carbocation Intermediates

The electrochemical generation of vinyl carbocations from alkenyl boronic esters and boronates is reported. Using easy-to-handle nucleophilic fluoride reagents, these intermediates are trapped to form fully substituted vinyl fluorides. Mechanistic studies support the formation of dicoordinated carbocations through sequential single-electron oxidation events. Notably, this electrochemical fluorination features fast reaction times and Lewis acid-free conditions. This transformation provides a complementary method to access vinyl fluorides with simple fluoride salts such as TBAF.

gem-Difluoroallylation of Aryl Halides and Pseudo Halides with Difluoroallylboron Reagents in High Regioselectivity

We report the palladium-catalyzed gem-difluoroallylation of aryl halides and pseudo halides with 3,3-difluoroallyl boronates in high yield with high regioselectivity, and we report the preparation of the 3,3-difluoroallyl boronate reactants by a copper-catalyzed defluorinative borylation of inexpensive gaseous 3,3,3-trifluoropropene with bis(pinacolato)diboron. The gem-difluoroallylation of aryl and heteroaryl bromides proceeds with low catalyst loading (0.1 mol % [Pd]) and tolerates a wide range of functional groups, including primary alcohols, secondary amines, ethers, ketones, esters, amides, aldehydes, nitriles, halides, and nitro groups. This protocol extends to aryl iodides, chlorides, and triflates, as well as substituted difluoroallyl boronates, providing a versatile synthesis of gem-difluoroallyl arenes that we show to be valuable intermediates to a series of fluorinated building blocks.

Gold N-Heterocyclic Carbene Catalysts for the Hydrofluorination of Alkynes Using Hydrofluoric Acid: Reaction Scope, Mechanistic Studies and the Tracking of Elusive Intermediates

An efficient and chemoselective methodology deploying gold-N-heterocyclic carbene (NHC) complexes as catalysts in the hydrofluorination of terminal alkynes using aqueous HF has been developed. Mechanistic studies shed light on an in situ generated catalyst, formed by the reaction of Brønsted basic gold pre-catalysts with HF in water, which exhibits the highest reactivity and chemoselectivity. The catalytic system has a wide alkyl substituted-substrate scope, and stoichiometric as well as catalytic reactions with tailor-designed gold pre-catalysts enable the identification of various gold species involved along the catalytic cycle. Computational studies aid in understanding the chemoselectivity observed through examination of key mechanistic steps for phosphine- and NHC-coordinated gold species bearing the triflate counterion and the elusive key complex bearing a bifluoride counterion.

Platinum Indolylphosphine Fluorido and Polyfluorido Complexes: An Interplay between Cyclometallation, Fluoride Migration, and Hydrogen Bonding

The reaction of [PtCl2 (COD)] (COD=1,5-cyclooctadiene) with diisopropyl-2-(3-methyl)indolylphosphine (iPr2 P(C9 H8 N)) led to the formation of the platinum(ii) chlorido complexes, cis-[PtCl2 {iPr2 P(C9 H8 N)}2 ] (1) and trans-[PtCl2 {iPr2 P(C9 H8 N)}2 ] (2). The cis-complex 1 reacted with NEt3 yielding the complex cis-[PtCl{κ2 -(P,N)-iPr2 P(C9 H7 N)}{iPr2 P(C9 H8 N)}] (3) bearing a cyclometalated κ2 -(P,N)-phosphine ligand, while the isomer 2 with a trans-configuration did not show any reactivity towards NEt3 . Treatment of 1 or 3 with (CH3 )4 NF (TMAF) resulted in the formation of the twofold cyclometalated complex cis-[Pt{κ2 -(P,N)-iPr2 P(C9 H7 N)}2 ] (4). The molecular structures of the complexes 1-4 were determined by single-crystal X-ray diffraction. The fluorido complex cis-[PtF{κ2 -(P,N)-iPr2 P(C9 H7 N)}{iPr2 P(C9 H8 N)}] ⋅ (HF)4 (5 ⋅ (HF)4 ) was formed when complex 4 was treated with different hydrogen fluoride sources. The Pt(ii) fluorido complex 5 ⋅ (HF)4 exhibits intramolecular hydrogen bonding in its outer coordination sphere between the fluorido ligand and the NH group of the 3-methylindolyl moiety. In contrast to its chlorido analogue 3, complex 5 ⋅ (HF)4 reacted with CO or the ynamide 1-(2-phenylethynyl)-2-pyrrolidinone to yield the complexes trans-[Pt(CO){κ2 -(P,C)-iPr2 P(C9 H7 NCO)}{iPr2 P(C9 H8 N)}][F(HF)4 ] (7) and a complex, which we suggest to be cis-[Pt{C=C(Ph)OCN(C3 H6 )}{κ2 -(P,N)-iPr2 P(C9 H7 N)}{iPr2 P(C9 H8 N)}][F(HF)4 ] (9), respectively. The structure of 9 was assigned on the basis of DFT calculations as well as NMR and IR data. Hydrogen bonding of HF and NH to fluoride was proven to be crucial for the existence of 7 and 9.

Reinvestigation of N,N-Diacetylimido-Protected 2-Aminothioglycosides in O-Glycosylation: Intermolecular Hydrogen Bonds Contributing to 1,2-Orthoamide Formation

N,N-Diacetylimido protection of 2-aminoglycosides is an elegant strategy but has had limited applications due to unexpected side reactions in glycosylation. We found that high acid concentrations could diminish the side reactions. We observed intermolecular hydrogen bonding among alcohols and acids could disrupt. Assuming that intermolecular hydrogen bonding accelerates the formation of 1,2-orthoamides and disrupting intermolecular hydrogen bonds could turn to the desired glycosylation, we successfully employed sulfenyl triflate pre-activation in the glycosylation of a broad scope of alcohol acceptors, as well as in a one-pot synthesis of a protected human milk oligosaccharide, lacto-N-neotetraose.

Chemo-, regio- and stereoselective synthesis of monofluoroalkenes via a tandem fluorination-desulfonation sequence

A widely applicable approach for the synthesis of Z-monofluoroalkenes from readily available alkyl triflones and NFSI has been reported. The reaction proceeded under mild conditions, affording mono-fluorinated alkenes in good to excellent yields with excellent chemo- regio- and stereoselectivity. The mechanism may involve electrophilic fluorination of triflones followed by the highly stereoselective concerted bimolecular elimination (E2) of CF3SO2H.

Palladium-catalyzed selective defluorinative arylation for the efficient stereospecific synthesis of (E)-β-fluoroacrylamides

A practical and efficient method for the synthesis of (E)-β-fluoroacrylamides was developed by the selective defluorinative arylation of trifluoropropanamides with arylboronic acids.

Gold-Catalyzed Intermolecular Alkyne Hydrofunctionalizations—Mechanistic Insights

An overview of the current state of mechanistic understanding of gold-catalyzed intermolecular alkyne hydrofunctionalization reactions is presented. Moving from the analysis of the main features of the by-now-generally accepted reaction mechanism, studies and evidences pointing out the mechanistic peculiarities of these reactions using different nucleophiles HNu that add to the alkyne triple bond are presented and discussed. The effects of the nature of the employed alkyne substrate and of the gold catalyst (employed ligands, counteranions, gold oxidation state), of additional additives and of the reaction conditions are also considered. Aim of this work is to provide the reader with a detailed mechanistic knowledge of this important reaction class, which will be invaluable for rapidly developing and optimizing synthetic protocols involving a gold-catalyzed alkyne hydrofunctionalization as a reaction step.

Pauson-Khand reaction of fluorinated compounds

The Pauson-Khand reaction (PKR) is one of the key methods for the construction of cyclopentenone derivatives, which can in turn undergo diverse chemical transformations to yield more complex biologically active molecules. Despite the increasing availability of fluorinated building blocks and methodologies to incorporate fluorine in compounds with biological interest, there have been few significant advances focused on the fluoro-Pauson-Khand reaction, both in the inter- and intramolecular versions. Furthermore, the use of vinyl fluorides as olefinic counterparts had been completely overlooked. In this review, we collect the advances both on the stoichiometric and catalytic intermolecular and intramolecular fluoro-Pauson-Khand reaction, with special attention to the PKR of enynes containing a fluoride moiety.

Stereodivergent Alkyne Hydrofluorination Using Protic Tetrafluoroborates as Tunable Reagents

The discovery of safe, general, and practical procedures to prepare vinyl fluorides from readily available precursors remains a synthetic challenge. The metal-free hydrofluorination of alkynes constitutes an attractive though elusive strategy for their preparation. Introduced here is an inexpensive and easily handled reagent that enables the development of simple and scalable protocols for the regioselective hydrofluorination of alkynes to access both the E and Z isomers of vinyl fluorides. These reaction conditions were suitable for a diverse collection of alkynes, including several highly functionalized pharmaceutical derivatives. Computational and experimental mechanistic studies support C-F bond formation through vinyl cation intermediates, with the E- and Z-hydrofluorination products forming under kinetic and thermodynamic control, respectively.